[llvm-readelf] - Allow dumping dynamic symbols when there is no program headers.

D62179 introduced a regression. llvm-readelf lose the ability to dump the dynamic symbols
when there is .dynamic section with a DT_SYMTAB, but there are no program headers:
https://reviews.llvm.org/D62179#1652778

Below is a program flow before the D62179 change:

1) Find SHT_DYNSYM.
2) Find there is no PT_DYNAMIC => don't try to parse it.
3) Print dynamic symbols using information about them found on step (1).

And after the change it became:

1) Find SHT_DYNSYM.
2) Find there is no PT_DYNAMIC => find SHT_DYNAMIC.
3) Parse dynamic table, but fail to handle the DT_SYMTAB because of the absence of the PT_LOAD. Report the "Virtual address is not in any segment" error.

This patch fixes the issue. For doing this it checks that the value of DT_SYMTAB was
mapped to a segment. If not - it ignores it.

Differential revision: https://reviews.llvm.org/D67078

llvm-svn: 371071