alpha: Do not build with -fpic

The combination of GCC 10 and binutils 2.35 (both unreleased) is no
longer able to link the dynamic linker, due to a GP16 relocation
overflow error:

glibc/alpha-linux-gnu/elf/librtld.os: in function `calloc': glibc/elf/../include/rtld-malloc.h:44:(.text+0xd98): relocation truncated to fit: GPREL16 against symbol `__rtld_calloc' defined in .data.rel.ro section in glibc/alpha-linux-gnu/elf/librtld.os
glibc/alpha-linux-gnu/elf/librtld.os: in function `malloc': glibc/elf/../include/rtld-malloc.h:56:(.text+0x2978): relocation truncated to fit: GPREL16 against symbol `__rtld_malloc' defined in .data.rel.ro section in glibc/alpha-linux-gnu/elf/librtld.os

This is arguably a linker bug; the object files and their section size
requirements look reasonable enough.

Using -fPIC (the default) works around this issue.