build: scope .NOTPARALLEL to locale targets only, restore parallel build

The locale Makefile (extra/locale/Makefile.in) had a bare ".NOTPARALLEL:"
directive (no prerequisites) since commit 4c68c74d ("locale: disable
parallel build as it is broken here", 2016-09-27).  That commit intended
to disable parallel execution only for the locale subdirectory build,
because of a race observed there at the time.

However, extra/locale/Makefile.in is `include`-d into the top-level
Makefile (Makefile.in:32), and the bare form of .NOTPARALLEL applies
globally to the make invocation, not just to the included fragment.
>From the GNU Make manual:

    If .NOTPARALLEL is mentioned as a target, then this invocation of
    make will be run serially, even if the -j option is given.

As a result, every uClibc-ng build since 2016 ran strictly serially,
regardless of -jN, across every architecture and target -- even on
multi-core build hosts.

Measured on arm-bbs-linux-uclibcgnueabihf, 16-core build host:

  before:  make -j16 cold build: 2m 18s real / 1m 38s user
                                   (user/real == 1.0  -- one core busy)
  after:   make -j16 cold build: 0m 42s real / 2m 14s user
                                   (user/real == 3.2  -- effective parallel)

  3.3x wall-clock speedup on a 16-core machine.

Fix: list the locale-generated headers and data files explicitly as
.NOTPARALLEL prerequisites.  This is the GNU Make 4.4+ form, where the
serial constraint is scoped to those specific targets (and their own
prerequisites).  GNU Make < 4.4 silently ignores the prerequisite list
and falls back to global serialization (i.e. the previous behavior),
so this change is fully backward-compatible -- new make versions get
the speedup, old make versions are unchanged.

The targets listed cover every recipe that writes to a generated file
via shell redirection (`> $@`, `-o $@`, etc.) inside the locale build,
preserving the original protection where the 2016 race was observed:

  - c8tables.h, wctables.h, locale_tables.h   (raw generator output)
  - locale_collate.h                          (depends on locale_tables.h
                                               via grep/sed pipeline)
  - lt_defines.h                              (greps locale_tables.h +
                                               locale_collate.h)
  - locale_data.c                             (gen_ldc output)
  - uClibc_locale_data.h                      (combines the above)
  - include/bits/uClibc_locale_data.h         (final install target)

The host-tool compilations (gen_wc8bit, gen_wctype, gen_collate,
gen_locale, gen_ldc) are intentionally not in the list -- they have no
inter-dependencies and benefit from parallel compilation.

Signed-off-by: Ramin Moussavi <ramin.moussavi@yacoub.de>

extra/locale/Makefile.in

@@ -5,7 +5,24 @@
 # Licensed under the LGPL v2.1, see the file COPYING.LIB in this tarball.
 #
 
-.NOTPARALLEL:
+# Scope .NOTPARALLEL to the generated locale headers/data only.  A bare
+# .NOTPARALLEL: (without prerequisites) used to be specified here, but
+# since this Makefile.in is `include`-d into the top-level Makefile, the
+# bare form serializes the *entire* uClibc-ng build, defeating make -jN
+# across every architecture, every target, since 2016.  The prerequisite
+# form (GNU Make 4.4+) limits the serial-only build to the locale data
+# generation, which is the original intent.  GNU Make < 4.4 silently
+# falls back to global serialization, i.e. previous behavior, so this
+# change is forward- and backward-compatible.
+.NOTPARALLEL: \
+	$(locale_OUT)/c8tables.h \
+	$(locale_OUT)/wctables.h \
+	$(locale_OUT)/locale_tables.h \
+	$(locale_OUT)/locale_collate.h \
+	$(locale_OUT)/lt_defines.h \
+	$(locale_SRC) \
+	$(locale_OUT)/uClibc_locale_data.h \
+	$(top_builddir)include/bits/uClibc_locale_data.h
 
 locale_DIR := $(top_srcdir)extra/locale
 locale_OUT := $(top_builddir)extra/locale