[lldb][AArch64] Add register fields for the fpmr register (#109934)

The FP8 formats have a "_" in the name so that they are:
1. Easier to read.
2. Possible to use in register expressions if/when they are supported.

Some other bits do have defined meanings but they are not simple to
name. Better that folks read the manual for those.

See this page for the full details:

https://developer.arm.com/documentation/ddi0601/2024-06/AArch64-Registers/FPMR--Floating-point-Mode-Register

lldb/test/API/linux/aarch64/fpmr/TestAArch64LinuxFPMR.py

@@ -45,6 +45,11 @@ class AArch64LinuxFPMR(TestBase):
             substrs=["Floating Point Mode Register", f"fpmr = {expected_fpmr:#018x}"],
         )
 
+        if self.hasXMLSupport():
+            self.expect(
+                "register read fpmr", substrs=["LSCALE2 = 42", "F8S1 = FP8_E4M3 | 0x4"]
+            )
+
         # Write a value for the program to find. Same fields but with bit values
         # inverted.
         new_fpmr = (0b010101 << 32) | 0b010