Add %lpad_hash for Zicfilp

Zicfilp has provided two labeling schemes: simple and complex (also known as
function signature-based). The simple scheme uses an lpad with a constant 0,
which does not require any hashing mechanism. In contrast, the complex
labeling scheme computes the MD5 hash from the signature string.

Filling up an MD5 hash value is straightforward for compilers, but it is
non-trivial work for humans to maintain. Therefore, we have added new assembler
modifiers to compute this value.

See also: riscv/riscv-cfi#151

riscv-asm.md

@@ -397,9 +397,10 @@ linker relaxation accidentally if user already disable linker relaxation.
 
 Push/pop current options to/from the options stack.
 
-## Assembler Relocation Functions
+## Assembler Modifiers
 
-The following table lists assembler relocation expansions:
+The RISC-V assembler supports the following modifiers for relocatable addresses
+or for computing special values used in RISC-V instruction operands:
 
 Assembler Notation          | Description                    | Instruction / Macro
 :----------------------     | :---------------               | :-------------------
@@ -410,11 +411,15 @@ Assembler Notation          | Description                    | Instruction / Mac
 %tprel_hi(symbol)           | TLS LE "Local Exec"            | lui
 %tprel_lo(symbol)           | TLS LE "Local Exec"            | load, store, add
 %tprel_add(symbol)          | TLS LE "Local Exec"            | add
-%tls_ie_pcrel_hi(symbol) \* | TLS IE "Initial Exec" (HI20)   | auipc
-%tls_gd_pcrel_hi(symbol) \* | TLS GD "Global Dynamic" (HI20) | auipc
-%got_pcrel_hi(symbol) \*    | GOT PC-relative (HI20)         | auipc
-
-\* These reuse %pcrel_lo(label) for their lower half
+%tls_ie_pcrel_hi(symbol) \*1 | TLS IE "Initial Exec" (HI20)   | auipc
+%tls_gd_pcrel_hi(symbol) \*1 | TLS GD "Global Dynamic" (HI20) | auipc
+%got_pcrel_hi(symbol) \*1    | GOT PC-relative (HI20)         | auipc
+%lpad_hash("func_signature") \*2 | Hash for landing pad labeling | lpad
+
+\*1 These reuse %pcrel_lo(label) for their lower half
+\*2 The format of the function signature string is defined in the RISC-V psABI.
+    The hash result is obtained from the lower 20 bits of the MD5 result of the
+    string.
 
 Labels
 ------------