libglib2: force arm mode when building with thumb1

Fixes #8491

glib/valgrind.h contains inline asm not compatible with thumb1, so build it
in arm mode:

  CC       libglib_2_0_la-gstringchunk.lo
/tmp/ccSVEXME.s: Assembler messages:
/tmp/ccSVEXME.s:587: Error: shifts in CMP/MOV instructions are only supported
in unified syntax -- `mov r12,r12,ror#3'
/tmp/ccSVEXME.s:587: Error: shifts in CMP/MOV instructions are only supported
in unified syntax -- `mov r12,r12,ror#13'
/tmp/ccSVEXME.s:588: Error: shifts in CMP/MOV instructions are only supported
in unified syntax -- `mov r12,r12,ror#29'
/tmp/ccSVEXME.s:588: Error: shifts in CMP/MOV instructions are only supported
in unified syntax -- `mov r12,r12,ror#19'
/tmp/ccSVEXME.s:589: Error: lo register required -- `orr r10,r10,r10'
Makefile:2087: recipe for target 'libglib_2_0_la-gslice.lo' failed

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>