[lld] Support thumb PLTs (#86223)

We are using PLTs for cortex-m33 which only supports thumb. More
specifically, this is for a very restricted use case. There's no MMU so
there's no sharing of virtual addresses between two processes, but this
is fine. The MCU is used for running [chre
nanoapps](https://android.googlesource.com/platform/system/chre/+/HEAD/doc/nanoapp_overview.md)
for android. Each nanoapp is a shared library (but effectively acts as
an executable containing a test suite) that is loaded and run on the MCU
one binary at a time and there's only one process running at a time, so
we ensure that the same text segment cannot be shared by two different
running executables. GNU LD supports thumb PLTs but we want to migrate
to a clang toolchain and use LLD, so thumb PLTs are needed.