net/openmpi: fix build on powerpc*

configure:16066: checking for short float
configure:16066: cc -c -DNDEBUG -O2 -pipe  -I/usr/local/include/gcc13 -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing  -finline-functions -isystem /usr/local/include conftest.c >&5
conftest.c:149:13: error: 'short float' is invalid
  149 | if (sizeof (short float))
      |             ^
1 error generated.

net/openmpi/Makefile

@@ -106,7 +106,7 @@ PLIST_SUB+=	NO_MCA_PATCHER_OVERWRITE="@comment "
 PLIST_SUB+=	NO_MCA_PATCHER_OVERWRITE=""
 .endif
 
-.if ${COMPILER_TYPE} == clang && ${COMPILER_VERSION} >= 170
+.if ${COMPILER_TYPE} == clang && ${COMPILER_VERSION} >= 170 && !${ARCH:Mpowerpc*}
 PLIST_SUB+=	SHORTFLOAT=""
 .else
 PLIST_SUB+=	SHORTFLOAT="@comment "