Reword fcsr ABI requirement

Author: sebhub

riscv-elf.md

@@ -72,7 +72,9 @@ f28-f31 | ft8-ft11     | Temporary registers    | No
 
 *: Floating-point values in callee-saved registers are only preserved across calls if they are no larger than the width of a floating-point register in the targeted ABI. Therefore, these registers can always be considered temporaries if targeting the base integer calling convention.
 
-The Floating-Point Control and Status Register (fcsr) is a thread-local register.
+The Floating-Point Control and Status Register (fcsr) must have thread storage
+duration in accordance with C11 section 7.6 "Floating-point environment
+<fenv.h>".
 
 # <a name=procedure-calling-convention></a> Procedure Calling Convention
 ## <a name=integer-calling-convention></a> Integer Calling Convention