Disable PIE for MIPS ubldr

When performing buildworld for MIPS with PIE enabled, the build fails
with "position-independent code requires '-mabicalls'" message.
-mno-abicalls and -fno-pic flags are explicitly set in MIPS ubldr
makefile, so to work around this problem, set MK_PIE=no for MIPS
ubldr.

Submitted by: Dawid Gorecki <dgr@semihalf.com>
Reviewed by: emaste
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D28370

stand/mips/uboot/Makefile

@@ -44,6 +44,10 @@ LDADD=		${LDR_INTERP} ${LIBUBOOT} ${LIBFDT} ${LIBUBOOT_FDT} ${LIBSA}
 
 OBJS+=  ${SRCS:N*.h:R:S/$/.o/g}
 
+# ubldr on MIPS is built with '-mno-abicalls' flag,
+# which is incompatible with PIE
+MK_PIE=		no
+
 ldscript.abs:
 	echo "UBLDR_LOADADDR = ${UBLDR_LOADADDR};" >${.TARGET}