[Committed] PR testsuite/105486: Use "signed char" in gcc.dg/pr102950.c

[Committed] PR testsuite/105486: Use "signed char" in gcc.dg/pr102950.c

Although the automated regression testing scripts for powerpc64 appear
to be somewhat garbled at the moment, they've correctly identified that
my new test case for pr102950.c is failing on powerpc64, as char by
default is unsigned on this target.  This patch tweaks the new testcase
by explicitly using "signed char" so that it's testing the intended EVRP
behaviour portably.  Committed as obvious.

2022-05-05  Roger Sayle  <roger@nextmovesoftware.com>

gcc/testsuite/ChangeLog
	PR testsuite/105486
	* gcc.dg/pr102950.c: Use explicit "signed char" in test case.