PhysX: Pin the glibc math symbols on x86-64 only

PhysX binds expf() and powf() to their GLIBC_2.2.5 versions, so that a
library built against a newer glibc still runs on an older one. That
version is the oldest glibc of x86-64; 32-bit x86 has the same
functions as GLIBC_2.0 and no GLIBC_2.2.5 version of them at all, so
once PhysX compiles for i686 with glibc, as the Yocto core2-32 builds
do, linking anything with it fails with undefined references to
powf@GLIBC_2.2.5.

Patch 0012 already took the architectures other than the Intel family
off this path, but the Intel family still has 32-bit x86 in it. Pin the
symbols on x86-64 alone, as patch 0021, which also leaves out
emscripten, since PhysX counts it as 32-bit x86.

Amends 7533abcc02b7f9bd7953203c489ee86d55e8c909

Fixes: QTBUG-151314
Assisted-by: Claude Code
Change-Id: Iacf64ce1faf03dddc940bcd4c2b14e619601f175
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>