Reduce value of LD_HWCAP_MASK for tst-env-setuid test case

Reduce value of LD_HWCAP_MASK for tst-env-setuid test case

It was discovered that the dynamic linker allocates a massive amount
of memory that increases with the value of LD_HWCAP_MASK.  Due to
this, setting its value to 0xffffffff in the environment of
tst-env-setuid would cause it to fail in some environments where
overcommit was disabled or severely constrained because malloc would
fail.

Since this test is only concerned with the value of LD_HWCAP_MASK
envvar being conserved (or not, for setxid binaries), lower its value
to avoid spurious failures.

The allocation bug is reported as #21502.