utils.c: On x86-32 unalign the stack before calling test_function

GCC when compiling with -msse2 and -mssse3 will assume that the stack
is aligned to 16 bytes even on x86-32 and accordingly issue movdqa
instructions for stack allocated variables.

But despite what GCC thinks, the standard ABI on x86-32 only requires
a 4-byte aligned stack. This is true at least on Windows, but there
also was (and maybe still is) Linux code in the wild that assumed
this. When such code calls into pixman and hits something compiled
with -msse2, we get a segfault from the unaligned movdqas.

Pixman has worked around this issue in the past with the gcc attribute
"force_align_arg_pointer" but the problem has resurfaced now in

    https://bugs.freedesktop.org/show_bug.cgi?id=68300

because pixman_composite_glyphs() is missing this attribute.

This patch makes fuzzer_test_main() call the test_function through a
trampoline, which, on x86-32, has a bit of assembly that deliberately
avoids aligning the stack to 16 bytes as GCC normally expects. The
result is that glyph-test now crashes.

V2: Mark caller-save registers as clobbered, rather than using
noinline on the trampoline.
pixman-glyph.c: Add __force_align_arg_pointer to composite functions

The functions pixman_composite_glyphs_no_mask() and
pixman_composite_glyphs() can call into code compiled with -msse2,
which requires the stack to be aligned to 16 bytes. Since the ABIs on
Windows and Linux for x86-32 don't provide this guarantee, we need to
use this attribute to make GCC generate a prologue that realigns the
stack.

This fixes the crash introduced in the previous commit and also

   https://bugs.freedesktop.org/show_bug.cgi?id=70348

and

   https://bugs.freedesktop.org/show_bug.cgi?id=68300
JOHNE - Presumably just line-ending changes??
Merge branch 'master' of git://anongit.freedesktop.org/git/pixman
JOHNE - Preliminary stuff for generating pkg-config files. Will probably get supplemented depending upon feedback from Todd