server/postgresql.mk: avoid gcc 11 ICE on armhf and armel

during RTL pass: mach
/<<PKGBUILDDIR>>/build/../src/backend/storage/buffer/bufmgr.c: In function ‘ReadBuffer_common’:
/<<PKGBUILDDIR>>/build/../src/backend/storage/buffer/bufmgr.c:1085:1: internal compiler error: in create_fix_barrier, at config/arm/arm.c:17891
 1085 | }
      | ^
0x106b1cf internal_error(char const*, ...)
	???:0
0x19f163 fancy_abort(char const*, int, char const*)
	???:0