package/libglib2: fix build on ARM in Thumb mode

package/libglib2: fix build on ARM in Thumb mode

Commit 4102db0f7 (package/libglib2: bump to version 2.60.3) did convert
libglib2 over to meson. In doing so, it left a very corner-case along.

When the target is an ARM CPU and the build is in thumb mode, then we
want to ensure that libglib2 is still built in arm mode (because of
inline asm).

But with meson, CFLAGS from the environment are passed to the host
compiler, so the build breaks, and the meson log contains:

    Appending CFLAGS from environment: '-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os   -marm'
    No LDFLAGS in the environment, not changing global flags.
    No CPPFLAGS in the environment, not changing global flags.
    Sanity testing C compiler: cc
    Is cross compiler: False.
    Sanity check compiler command line: cc -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -marm [...]/build/libglib2-2.60.4/build/meson-private/sanitycheckc.c -o [...]/build/libglib2-2.60.4/build/meson-private/sanitycheckc.exe
    Sanity check compile stdout:
    -----
    Sanity check compile stderr:
    cc: error: unrecognized command line option ‘-marm’; did you mean ‘-mabm’?
    -----
    meson.build:1:0: ERROR: Compiler cc can not compile programs.

Fix that by using the new per-package CFLAGS feature of the meson infra.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Cc: Adam Duskett <aduskett@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>