lldb/ObjectFile,Disassembler: read some state from the executable

Add support to inspect the ELF headers for RISCV targets to determine if
RVC or RVE are enabled and the floating point support to enable.  As per
the RISCV specification, d implies f, q implies d implies f, which gives
us the cascading effect that is used to enable the features when setting
up the disassembler.  With this change, it is now possible to attach the
debugger to a remote process and be able to disassemble the instruction
stream.

~~~
$ bin/lldb tmp/reduced
(lldb) target create "reduced"
Current executable set to '/tmp/reduced' (riscv64).
(lldb) gdb-remote localhost:1234
(lldb) Process 5737 stopped
* thread #1, name = 'reduced', stop reason = signal SIGTRAP
    frame #0: 0x0000003ff7fe1b20
->  0x3ff7fe1b20: mv     a0, sp
    0x3ff7fe1b22: jal    1936
    0x3ff7fe1b26: mv     s0, a0
    0x3ff7fe1b28: auipc  a0, 27
~~~