[Bug tdep/28653] New: "set arch armv7e-m" doesn't actually make gdb think it's connected to an m-profile target

["cjn at google dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28653

            Bug ID: 28653
           Summary: "set arch armv7e-m" doesn't actually make gdb think
                    it's connected to an m-profile target
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: tdep
          Assignee: unassigned at sourceware dot org
          Reporter: cjn at google dot com
  Target Milestone: ---

"arm_psr_thumb_bit" and "arm_pc_is_thumb" rely on the "is_m" flag being set
inside "arm_gdbarch_init".

That only can happen if an m-profile executable file is loaded or a complete
target description is available, though. "arm_gdbarch_init" doesn't actually
pay attention if explicitly told: "set archicecture armv7e-m" - the "is_m" flag
remains zero, and so software single stepping can be weird, and the register
dumps show "cpsr" rather than the expected "xpsr".

I added a print where the "tdep->is_m" flag is set near the end of
"arm_gdbarch_init" and get this output:

    (gdb) set architecture armv7e-m
    arm_gdbarch_init:9293: is_m = 0
    The target architecture is set to "armv7e-m".

Setting the architecture indirectly via a "file" command, does work around the
problem:

    (gdb) show architecture
    The target architecture is set to "auto" (currently "i386").
    (gdb) file example.elf
    arm_gdbarch_init:9293: is_m = 1
    Reading symbols from example.elf...
    (gdb) show architecture
    The target architecture is set to "auto" (currently "armv7e-m").

There are other m-profile architectures available via "set architecture" that
should also be fixed.

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