[dev.link] cmd/link: reduce alignment requirement for symtab/elfstr
The linker's asmb phase for ELF has a chunk of code that decides where
to place the .symtab and .strtab sections, which appear after after
the DWARF data; this code currently tries to align the start of the
.symtab section using the value of -R (stored in *FlagRound). This
patch gets rid of this additional alignment and instead just aligns
.symtab by pointer size. The -R value is needed for loadable
segments/sections (such as text or data), not for non-loadable
sections (e.g. symtab). On most architectures the *FlagRound value is
4k, however on ARM64 it is 64k, meaning that aligning symtab on this
boundary can waste a good chunk of space.