MFC r361740: lld: Set DF_1_PIE for -pie

DF_1_PIE originated from Solaris[1].

GNU ld[2] sets the flag on non-Solaris platforms.

It can help distinguish PIE from ET_DYN.
eu-classify from elfutils uses this to recognize PIE[3].

glibc uses this flag to reject dlopen'ing a PIE[4]

[1] https://docs.oracle.com/cd/E36784_01/html/E36857/chapter6-42444.html
[2] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=5fe2850dd96483f176858fd75c098313d5b20bc2
[3] https://sourceware.org/git/?p=elfutils.git;a=commit;h=3f489b5c7c78df6d52f8982f79c36e9a220e8951
[4] https://sourceware.org/bugzilla/show_bug.cgi?id=24323

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