[ELF] Make dot in .tbss correct

GNU ld doesn't support multiple SHF_TLS SHT_NOBITS output sections (it restores
the address after an SHF_TLS SHT_NOBITS section, so consecutive SHF_TLS
SHT_NOBITS sections will have conflicting address ranges).

That said, `threadBssOffset` implements limited support for consecutive SHF_TLS
SHT_NOBITS sections. (SHF_TLS SHT_PROGBITS following a SHF_TLS SHT_NOBITS can still be
incorrect.)

`.` in an output section description of an SHF_TLS SHT_NOBITS section is
incorrect. (https://lists.llvm.org/pipermail/llvm-dev/2021-July/151974.html)

This patch saves the end address of the previous tbss section in
`ctx->tbssAddr`, changes `dot` in the beginning of `assignOffset` so
that `.` evaluation will be correct.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D107208

(cherry picked from commit 9bd29a73)